A fuel supply system is known, which includes both a fuel injection valve that injects a fuel to be burned in an internal combustion engine and a fuel pump that pumps the fuel to the fuel injection valve. The fuel supply system has sliding portions that slide while being exposed to the fuel, such as, for example, a sliding portion of the fuel pump and a sliding portion of the fuel injection valve, and in such sliding portions, the fuel functions as a lubricant. Therefore, if a fuel having low lubricity is used, there is a high risk that either or both of the sliding portions may be damaged, which may result in a sliding failure.
The fuel having low lubricity is one having a low retention property to a sliding portion, such as, for example, a fuel having a low dynamic viscosity. Patent Document 1 discloses a dynamic viscosity sensor that detects the dynamic viscosity of a fuel, and it can be estimated that as the dynamic viscosity thus detected is lower, a fuel has lower lubricity.
A fuel contains components having plural types of molecular structures, such as components having molecular structures of aromas and components having molecular structures of paraffins. The mixing ratios of these components differ depending on the mining location, refinery location, and the like of fuels. When fuels are compared with each other at such a molecular structure level, lubricity may differ when the mixing ratio of each molecular structure species differs, even if the detected dynamic viscosities are equal to each other. Therefore, when it is intended to estimate lubricity from the dynamic viscosity detected by a dynamic viscosity sensor, there is a limit in improving the estimation accuracy.